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Verificationism. Classical Empiricism. Last time we learned about the idea theory. Although it wasn’t confined to the empiricists, most of them ascribed to it (Locke, Berkeley, and Hume being the most notable).

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classical empiricism
Classical Empiricism

Last time we learned about the idea theory. Although it wasn’t confined to the empiricists, most of them ascribed to it (Locke, Berkeley, and Hume being the most notable).

Empiricism was variously the doctrine that all ideas “came from” experience, or that all knowledge did, or both. (Usually both.)

classical empricism
Classical Empricism

Empiricism had its problems, in addition to those that the idea theory suffered from:

Experience tells you what is, not what must be/ should be/ will be. Yet we can know some of these things.

Poverty of the stimulus: We figure out things like language use faster than experience is capable of teaching us. This suggests innateness.


The French philosopher/ first Western sociologist Isidore Auguste Marie François Xavier Comte (1798-1857) theorized that society progressed in three stages: from the theological, to the metaphysical, to the “positive.”


In the theological stage, people believe any silly or magical thing their ancestors attributed to the gods. Next, in the metaphysical stage gods go out of the picture, but are replaced with unjustified “metaphysical” assumptions (e.g. universal human rights). Finally, in the positive stage, the truth of our beliefs is “positively” determined. Compte thought science was the only source of positive determination.

logical empiricism positivism
Logical Empiricism/ Positivism

Around the 1920’s in Vienna and Berlin certain philosophical doctrines became popular, and their adherents were variously known as Logical Empiricists or Logical Positivists (sometimes neo-Positivists). Notable names included Rudolf Carnap, Hans Reichenbach, and A.J. Ayer.

empiricist criterion of cognitive significance
Empiricist Criterion of Cognitive Significance

According to the logical positivists, in order for a sentence to have cognitive significance (to be meaningful), it had to have verification conditions.

(‘Verification’ is a Latinate English word < ‘veri-’ true + ‘facere’ to make. Verification conditions are conditions under which the truth of a statement can be conclusively established.)

empiricist criterion of cognitive significance1
Empiricist Criterion of Cognitive Significance

In fact, the positivists maintained that the meaning of a sentence was its verification conditions. So a sentence with no verification conditions– where no experience can establish its truth– is meaningless.

truth vs verification
Truth vs. Verification

Many philosophers (even today) have identified the meaning of a sentence with its truth conditions. These are the circumstances in which the sentence would be true. But the positivists went farther– they held that the meaning of a sentence was its verification conditions– the circumstances in which we would know the sentence was true.

the elimination of metaphysics
The Elimination of Metaphysics

This was part of a radical philosophical agenda, which included “the elimination of metaphysics.” The idea was to view many philosophical problems of the past (and also many religious claims) as meaningless disputes that could simply be ignored.

the elimination of metaphysics1
The Elimination of Metaphysics

Example: In a religion where God is beyond human experience, the positivists would say that “God exists” is neither true nor false but meaningless, since no experience could verify it.

Kant, Hegel, and Heidegger were also big targets for the positivists. Example Hegel quote: “But the other side of its Becoming, History, is a conscious, self-meditating process — Spirit emptied out into Time.”

the elimination of metaphysics2
The Elimination of Metaphysics

The positivists even wanted to eliminate a lot of more down-to-Earth metaphysics:

Modality: We can only experience what is, not what could possibly be. So statements about what is (merely) possible are meaningless.

Normativity: We can only experience what is, not what should morally be. So statements about what is good or bad are meaningless.

the new science
The New Science

There was also a scientific impetus to logical positivism (beyond the just pro-science message of positivism). Kant influentially held that Euclidean geometry was synthetic a priori, and that our experience must be as of a Euclidean spacetime. But the Minkowskispacetime in relativity is non-Euclidean.


How do you respond to opponents (classical physics) that think their theory is knowable in advance of any argument or evidence? Einstein responded by operationalizing: imagining rigid rods extending in all directions, and clocks at various points. That is, his arguments were couched in terms of what you could measure or experience (rather than straightforwardly in terms of what was true).

quantum mechanics
Quantum Mechanics

Quantum mechanics also had metaphysical problems of its own. Several counterintuitive experiments seemed to suggest that the basic laws of the universe were not quite consistent with the laws of logic (e.g. the distribution laws). This led some physicists to simply deny that there were questions to be answered beyond “what do we observe/ experience?”– no questions like “what is the reality causing the appearances?”

empiricist semantics
Empiricist Semantics

According to the positivists, the elimination of metaphysics followed from the correct account of meaning. When we understood that meaning = verification conditions, then we would see that ‘the Absolute is perfect’ or ‘God exists’ can’t possibly have meanings. Then we would be free to look into more promising, resolvable philosophical questions.

protocol sentences
Protocol Sentences

We single out a certain, small set of sentences to be the “protocol” or “observation” sentences. These sentences are all very simple syntactically, along the lines of: ‘that is red.’

The importance of the protocol sentences is that they can be immediately verified. To tell whether ‘that is red’ is verified (is true), you just have to look.

non protocol sentences
Non-Protocol Sentences

All the other meaningful sentences (according to the verificationist) are defined in terms of the protocol sentences and the logical vocabulary (AND, OR, NOT, ALL, SOME, NO, etc.).

For example ‘That is an arthropod’ := That is an animal AND it has a jointed body AND it has segmented legs.

non protocol sentences1
Non-Protocol Sentences

Obviously these sorts of definitions work best with scientific terminology like ‘arthropod,’ but the positivists were happy with that. It could turn out that much of our ordinary talk was not strictly speaking meaningful, but needed to be regimented in a more scientific language.

protocol sentences1
Protocol Sentences

There was some measure of debate among the positivists regarding which sentences actually qualified as protocol sentences. The simpler the qualities they are about (e.g. ‘that is red’ ‘that is warm’ ‘this is joy’) the easier it is to argue that they can be verified immediately, but the harder it is to define the rest of the sentences (try defining “Obama is the president of the US” in terms of what things are red, warm, joy, etc.!

protocol sentences2
Protocol Sentences

On the other hand, it’s easier to define more abstract things if we let sentences like ‘That is a chair’ or ‘That is a person’ be protocol sentences. However, can these things really be immediately verified? Our observations don’t seem to guarantee that something is a chair (it might be a fake chair, or the reflection of a chair, or…)

carnap s aufbau

In the Aufbau (The Logical Structure of the World), Carnap undertook an ambitious project to outline how one could translate all “high-level” talk (e.g. “the train to Vienna is running late”) into talk about sensations at coordinate points in the visual field (“quality q is at point-instant x;y;z;t” [actually it was even more ambitious]). I should note that Carnap himself wasn’t much of a fan of the Aufbau after completing it.

verificationist semantics
Verificationist Semantics

So here’s the picture: The meaning of a sentence is the set of experiences that would verify it.

Protocol (observation) sentences are directly connected with their verification conditions: we can immediately tell whether they are verified in any particular circumstance.

Non-protocol sentences inherit their verification conditions from the protocol sentences they are logically constructed out of.

special exception
Special Exception

One exception was made: logic and mathematics were held to be meaningful, even though its hard to state (for example) what experiences would confirm “2 + 2 = 4.” Intuitionism (constructivism) was a positivist-influenced, non-classical approach to logic and mathematics that said that only provable formulas (only “mathematically verifiable” formulas) were true (denial of excluded middle).

the idea theory
The Idea Theory

Here were the essential parts of the idea theory:

  • Words and sentences are the visible, conventional signs of ideas.
  • Ideas represent things in the world by resembling (non-conventional) them.
  • We can treat the meaning of a word as either the idea it is a sign of, or the thing that idea represents (it doesn’t really matter).

Verificationism is similar. A word or a sentence was conventionally associated with a set of experiences. Those experiences verify (“make true,” a non-conventional relation) that things in the world are a certain way because of a perfect correlation between the experiences and the states they verify. Sometimes this correlation was enforced by an idealist worldview, or a view on which the external world was logically constructed from sense data.

representation is not an equivalence relation
Representation is Not an Equivalence Relation

A main problem for the idea theory was its identification of representation with resemblance. While resemblance is an equivalence relation (reflexive, symmetric, and transitive), representation is not.


But the verificationist thinks that the meaning of a sentence is the experience(s) that would verify it. Thus it follows (correctly) that most sentences do not represent themselves, because most sentences don’t verify themselves (exception: the sentence “This is a sentence”).

abstract concepts
Abstract Concepts

Idea theorists, as we saw, also had a problem with abstract ideas, as no picture equally resembles a fat man and a skinny man. One potential line of response we saw was to let abstract ideas be a collection of ideas, and to say that the collection represented what is common (shared by) all the items in the collection.

abstract concepts1
Abstract Concepts

The verificationist strategy was similar: the meaning of ‘x is a dog’ was the collection of experiences that would verify it. However, these experiences need not share any features: one might be an experience I’d have with my eyes closed (shaggy coat feeling, barking noises), and another might involve visual impressions of a dog. Both could verify ‘x is a dog.’

only so much time
Only So Much Time…

Obviously, I can’t re-cover every objection we considered to the idea theory and then see how the verificationist does with respect to that question.

BUT, that doesn’t mean that wouldn’t be an interesting paper topic for you to do.

theoretical concepts
Theoretical Concepts

One last issue though. Verificationism was thought to have particular trouble with theoretical concepts (that is, with representing theoretical entities) like electrons or DNA. (These are called “theoretical entities” because we can’t observe them directly, but their existence is confirmed by their characteristic effects as described by our scientific theories. Example: effects of charged particles in cloud chambers.)

idea theory vs theoretical entities
Idea Theory vs. Theoretical Entities

The idea theory had it pretty bad with respect to such entities, but no one who lived at the time the idea theory was big knew this. For all anybody knew, particles were very much like mental pictures of little tiny pool balls. But now its implausible to think that anyone’s “mental images” of an electron come close to resembling electrons.

verificationism vs theoretical entities
Verificationism vs. Theoretical Entities

Verificationism was fine here, at least insofar as you could say that the behavior of the gas in the cloud chamber verified the existence of electrons, even though it didn’t resemble them.

the problem
The Problem

The problem was that the meanings of scientific terms was supposed to be fixed in advance. Yet for many theoretical terms, it took years or decades after their introduction for us to discover any way of verifying claims about them. (Compare: ‘x is a Higgs Boson.’) So did claims about electrons, positrons, mesons, or whatever not mean anything until we discovered ways of verifying them. And did we discover their meanings then?

too little is meaningless
Too Little is Meaningless

The logical empiricists wanted to say that sentences like “The Absolute is Perfect” and “God exists” are meaningless. If you’re of that persuasion, you’re likely to think that “Either some socks are cotton or the Absolute is Perfect” and “Either God exists or snow is purple” are also meaningless. But the latter two clearly have conditions that would verify them.

too much is meaningless
Too Much Is Meaningless

A bigger focus of criticism, however, was that according too the verifiability criterion, too much is meaningless, including:

  • Statements about the past or future.
  • Negative existentials.
  • Positive universals.
  • Certain positivist doctrines.
statements about the past future
Statements about the Past/ Future

One objection to the verifiability criterion was that it made statements about the distant past or the distant future meaningless, since there is no way of verifying, for example, the statement “T. Rex had a blue tongue” or “Hats will be popular among the first humans that colonize Alpha Centauri.”

a confusion
A Confusion

This objection is a little bit confused. Positivists don’t claim that for any meaningful sentence, there actually exists evidence you could find that would (when you found it) confirm that sentence. This would imply that every meaningful sentence was true. To be meaningful, a sentence just has to have verification conditions– it has to be possible for there to be circumstances that verify it.

a confusion1
A Confusion

So I could, possibly, verify that T. Rex had a blue tongue by finding a perfectly preserved frozen T. Rex with a blue tongue.

Sure, that won’t happen, but that’s not the point. Compare “God exists”– here, no experience will verify that claim, not even possible experience.


However, this response only goes so far. What sort of evidence now could conclusively show that hats will be popular on Alpha Centauri?

Additionally, we can reformulate the objection. Events outside my light-cone cannot affect me. So in what sense is it even possible to verify “A dinosaur outside my light-cone has a blue tongue”?

verifiability in principle
Verifiability “In Principle”

However, the objection isn’t so simple: complex sentences like this were supposed to be built out of protocol sentences like ‘x is a T. Rex’ and ‘x is blue’ and ‘x is a tongue.’ Each of these has verification conditions. So we can say that a sentence is verifiable in principle if it is a logical construct out of protocol sentences, each of which is verifiable in the old sense.

verifiability in principle1
Verifiability “In Principle”

Russell pointed out however that some statements that seem meaningful are not verifiable in principle:

  • Neptune existed before it was discovered.
  • Atomic war will kill everyone.

Someone couldn’t verify (1) before the discovery of Neptune, and they couldn’t verify it afterward either! Similar remarks go for (2).

the verifiability criterion itself
The Verifiability Criterion Itself

Consider the verifiability criterion: “a sentence is meaningless unless some finite procedure can conclusively verify its truth.” If this criterion is meaningful, then it must be that some finite procedure can conclusively verify the claim that a sentence is meaningless unless some finite procedure can conclusively verify its truth. But what procedure would that be? (Also note that the criterion doesn’t meet the special exception, because it’s not a logical truth either.)

kicking away the ladder
Kicking Away the Ladder

“My propositions serve as elucidations in the following way: anyone who understands me eventually recognizes them as nonsensical, when he has used them - as steps - to climb beyond them. He must, so to speak, throw away the ladder after he has climbed up it.” (Wittgenstein, TractatusLogico-Philosophicus, 6.54)

existentials and universals
Existentials and Universals

Here’s a(n incomplete) typology of claims:

Positive existential: There is an F that is G.

Negative existential: There is no F that is G.

Positive universal: Every F is G.

Negative universal: Not every F is G.

existentials and universals1
Existentials and Universals

Positive existential claims and negative universal claims can be verified by a finite number of experiences. For instance, it suffices to observe just one cow that is dangerous to know that:

  • There is a cow that is dangerous.
  • Not every cow is safe.
existentials and universals2
Existentials and Universals

However, negative existentials and positive universals cannot be verified by a finite number of claims. If I observe one billion cows that are dangerous, I still have not shown conclusively:

  • There is no cow that is safe.
  • All cows are dangerous.
negative existentials
Negative Existentials

Russell tells the following story: “[Wittgenstein] maintained, for example, at one time that all existential propositions are meaningless. This was in a lecture room, and I invited him to consider the proposition: 'There is no hippopotamus in this room at present.' When he refused to believe this, I looked under all the desks without finding one; but he remained unconvinced.”


We might choose to instead identify the meaning of a sentence with its falsification conditions (rather than its verification conditions). These are the circumstances under which it can conclusively be known to be false.


This resolves the negative existentials and and positive universals. Observing one safe cow is enough to falsify:

  • There is no cow that is safe.
  • All cows are dangerous.

However, this just turns the tables: now we can’t handle positive existentials or negative universals: even observing one billion cows that are safe, I still have not falsified with certainty:

  • There is a cow that is dangerous.
  • Not all cows are safe.
falsificationism too loose
Falsificationism too Loose

Additionally, since both “The absolute is perfect and some socks are made of cotton” and “Snow is purple and God exists” are falsifiable, they turn out perfectly meaningful for the falsificationist.

It seems as though falsificationism is no better off than verificationism.


Would it help to say:

New Criterion: A sentence S is only meaningful if one or more of the following is true:

  • S is a logical truth.
  • S is verifiable.
  • S is falsifiable.
all over some
All over Some

Surprisingly, that would not help. Consider the sentence, “For any substance, there exists a solvent,” and consider some set of observations:

  • A dissolves X
  • B dissolves X
  • C dissolves Y
  • C dissolves Z
neither verifiable nor falsifiable
Neither Verifiable Nor Falsifiable

No matter how many observations you pile up, the sentence “For any substance, there exists a solvent” will not be verified conclusively. Because there may always be a substance you have not yet considered that does not have a solvent. Furthermore, no set of observations will falsify the sentence, because even if you have yet to find a solvent for substance S, there may always be one you have not yet considered.


One move to lessen some of the negative implications of verificationismwas to deny that the meaning of a sentence was the conditions under which it was verified (or falsified), and instead identify it with the conditions under which the statement was confirmed (or “infirmed”).

the basic idea
The Basic Idea

So even if nothing conclusively guaranteed the truth or falsity of “for every substance, there exists a solvent,” the fact that you’d found a billion substances, and a solvent for each one, strongly confirmed that hypothesis. Or the fact that you searched for a billion years extensively, and have yet to find solvents for over a billion substances strongly disconfirmed (“infirmed”) it.

nice try
Nice Try

This was a nice suggestion, but ultimately it failed too, and American philosopher W.V.O. Quine is widely recognized to have put the nail in the coffin of verificationism/ confirmationism in his landmark essay “Two Dogmas of Empiricism.”

two dogmas
Two Dogmas

By the time Quine wrote “Two Dogmas of Empiricism” in 1951, logical positivism was pretty unpopular. Still, “Two Dogmas” was insanely influential, and is easily one of the most important works of 20th Century “analytic” philosophy.

analytic synthetic distinction
Analytic/ Synthetic Distinction

Quine’s first attack is on the dogma (of empiricism) that there’s a distinction between analytic and synthetic truths– things that are true because of what they mean (“vixens are female foxes”) and things that are true because of the facts (“there are vixens in our back yard”).

non central

As we’ve seen, the positivists assumed such a distinction, but there’s no reason to think that it’s central to verificationism/ confirmationism. In fact, I’ve characterized the positivist treatment of logical and mathematical truths as a “special exception”– they wouldn’t need a special exception if they admitted, with Quine, that nothing is wholly analytic.

the second dogma
The Second Dogma

The good stuff starts in Section 5, “The Verification Theory and Reductionism.” Here, the dogma at issue is verificationism(how we’ve been using the terminology: confirmationism). Quine also calls it ‘radical reductionism’ because it maintains that every sentence ultimately has a meaning that is just a set of personal experiences. It “reduces” statements about chairs and tables, to statements about personal experiences.

the quine duhem thesis
The Quine-Duhem Thesis

“[It’s not true] that each statement, taken in isolation of its fellows can admit of confirmation or infirmation… Our statements about the external world face the tribunal of sense experience not individually but as a corporate body.” (“Two Dogmas,” p. 41).

“The unit of empirical significance is the whole of science.” (“Two Dogmas,” p. 42).

the quine duhem thesis1
The Quine-Duhem Thesis

Here’s the general picture. Suppose I have an experience as of seeing brick houses on Elm Street. Quine is claiming that this does not by itself confirm the statement “There are brick houses on Elm Street.” If I had strong prior beliefs that, for instance, all bricks had been destroyed and that I’d been given a powerful hallicinogen that made me see brick houses, then I would not assign a higher probability to this claim.

underdetermination of theory by data
Underdetermination of Theory by Data

Our observational data (experiences) underdetermine our theories (claims about the extra-mental world). My experiences as of brick houses are compatible with infinitely many theories (actual brick houses, hallucination, computer simulation, visual error…). Only my entire body of beliefs (that I’m not hallucinating, that my eyes are working properly, that I seem to be seeing brick houses) confirms or disconfirms a theoretical statement.

ubiquity of underdetermination
Ubiquity of Underdetermination

This is not just reserved for “skeptical” scenarios. Lots of evidence seems to confirm the thesis that the Earth does not move: we aren’t, for instance, thrown off of it as it hurtles through space. The individuals in the past who took this as evidence for a stationary Earth had a mistaken theory of motion. Against the background of modern theory, this is not confirmatory evidence at all.

old problem
Old Problem

This is actually the same problem as we considered before: verificationism vs. theoretical statements. The reason we discover methods of verification, rather than stipulate them in advance, is that our theories advance, and according to the new theories, certain experiences confirm certain phenomena. If our theories change, those same experiences may no longer confirm those same phenomena.

the web of belief
The Web of Belief

Quine’s picture is that our beliefs form a “web” where change in the degree of belief in any statement affects the degrees of belief in all of the others, simultaneously. Some statements are more toward the “periphery” of the web (observation statements), and they are more likely to change with changing experience. But sometimes “recalcitrant” experience causes us not to revise the periphery, but the more central, deeply theoretical (and even logical) statements.


Verificationism is the view that the meaning of a statement is the conditions under which it is verified– the experiences such that if you have them, then the statement is true.

aims and influence
Aims and Influence

The project was influential: both influenced by and influencing the science of the time. It advocated the radical elimination of all metaphysics and normativity (art, morality) in favor of a regimented, operationalist science.

early problems
Early Problems

Early problems for verificationism included:

  • It couldn’t make sense of the discovery of methods of verification for theoretical entities.
  • It seemed incompatible with meaningful statements about things not possible to experience.
  • It threw out various clearly scientifically acceptable claims “For every substance…”

Many positivists thought that the limitations of verificationism could be overcome by generalizing the idea to confirmationism: the meaning of a statement is the conditions under which it is confirmed– the experiences such that, if you have them, the statement is more likely to be true.

the quine duhem thesis2
The Quine-Duhem Thesis

However, philosophers of science eventually came to accept that confirmation is theory-dependent, meaning that it made no sense to ask what experiences confirm or disconfirm statement X– only what experiences confirm or disconfirm X relative to theory T.


It was thus possible to continue to maintain confirmationism, at the cost of holism. If confirmation is theory-dependent, and meaning = confirmation conditions, then meaning is theory dependent: two people with different theories cannot mean the same thing by the same statement. Different philosophers differ in their taste for such holism.